Acoustic-internal wave interaction at long ranges in the ocean

Abstract

Fluctuations in the amplitude and phase of low-frequency sound propagated to long range in the ocean are predicted. Phase fluctuations are attributed to the passage of acoustic radiation through the internal gravity-wave field; predictions are based on measured and modeled internal wave spectra. Ray theory is used to determine phase and amplitude variations as a function of time, space, and acoustic frequency. It is shown and experimentally verified that mean-square phase fluctuations are depth dependent.Subject Classification: 30.20. A b E(ot, w•.) ß g(z), M no, R,T S acoustic signed amplitude internal-wave stratification depth sound velocity internal-wave energy density internal-wave displacement, acoustic thickness, and phase spectral densities acceleration of gravity horizontal distance, travel time from depth z to the r•earest upper turning point eigenray order complex ampiRude of the acousticthickness wave internal-wave peak buoyancy frequency, buoyancy-frequency depth distribution acoustic ray cycle length, travel time acoustic-internal-wave correlation factor temperature, adiabatic temperature, potential temperature, and salinity gradients internal-wave period Z(z) B, ß 6o 0 p, p•, 0), O.)i, O)g Turner number or temperature-salinity relationship normalized wave function internal-wave horizontal wavenumber linearized salinity, temperature coefficients from Wilson's equation {ndex of refraction sound velocity/internal wave interaction parameter internal-wave displacement Dirac delta function time-and space-varying acoustic thickness acoustic phase acoustic, internal wavelength wavenumber cutoff coefficient potential density horizontal acoustic ray angie in interhal-wave layer acoustic field acoustic, inertial, internal-wave frequency

RETRACTED: The Effects of Internal Waves on Propagation Behavior of Sound in the Sea

“…The internal wave displacement can be written as [6]: × that we also have used same value in our computations. Defining a displacement spectral density ( )…”

“…On the basis of myriad observations, Garrett and Munk have contrived successive models (GM72, GM75) of internal wave spectra. We use the GM75 spectrum, somewhat modified for the Cairns observations [6] [7]:…”

Temporal coherence of the acoustic field forward propagated through a continental shelf with random internal waves

“…The bottom sediment half-space is composed of sand with density d b ¼ 1.9 g/cm 3 and sound speed c b ¼ 1700 m/s. The attenuation coefficients in the water column and bottom are a ¼ 6 Â 10 À5 dB/k and a b ¼ 0.8 dB/k, respectively.…”

“…[1][2][3][4][5][6][7] The acoustic coherence time scale is an important parameter in remote sensing applications because it determines (i) the time window within which standard coherent processing, such as matched filtering, beamforming, and synthetic aperture processing may be conducted, and (ii) the number of statistically independent fluctuations in a given measurement period that determines the variance reduction possible by stationary averaging. 8,9 To model and characterize acoustic field fluctuations, earlier work in the 1970s and 1980s focused on ray theory to estimate temporal coherence by accumulating random phase fluctuations along isolated water-borne ray paths in the deep ocean.…”